This invention relates to a mechanical plug drill for removing plugs from plugged tubes in a steam generator.
A typical nuclear steam generator comprises a vertically oriented shell, a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle, a tube sheet for supporting the tubes at the ends opposite to the U-curvature, and a dividing plate that cooperates with the tube sheet forming a primary fluid inlet plenum at one end of the tube bundle and a primary fluid outlet plenum at the other end of the tube bundle. The area below the tube sheet formed by the inlet and outlet plenums is the channel head where some of the repair and maintenance work on the steam generator is performed.
The primary fluid having been heated by circulation through the nuclear reactor core enters the steam generator through the primary fluid inlet plenum. From the primary fluid inlet plenum, primary fluid flows upwardly through the first openings in the U-tubes near the tube sheet which supports the tube, through the U-tube curvature, downward through the second openings in the U-tubes near the tube sheet, and into the primary fluid outlet plenum. At the same time, a secondary fluid known as feedwater is circulated around the U-tubes in heat transfer relationship therewith, thereby transferring heat from the primary fluid in the tubes into the secondary fluid surrounding the tubes causing a portion of the secondary fluid to be converted to steam.
Since the primary fluid contains radioactive particles and is isolated from the secondary fluid by the U-tube walls and the tube sheet, it is important that the U-tubes and the tube sheet be maintained defect-free so that no breaks will occur in the U-tubes or in the walls between the U-tubes and the tube sheet, thus preventing contamination of the secondary fluid by the primary fluid.
The tubes in steam generators are subjected to flow induced vibration, chemical corrosion, as well as temperature gradients, all of which cause the tubes to degrade. When certain of the tubes in the tube bundle degrade due to the above set forth factors, it is necessary to plug the degraded tubes in order to prevent contamination of the secondary fluid by the primary fluid. However, in some cases it is desirable to unplug the plugs either for repair or for sleeving. Sleeving is a process whereby the defects in the tube wall are cured by inserting a sleeve thereinto, thereby enabling the tube to be put back into service. Various types of plugs are used to plug degraded tubes in a steam generator, but whatever type of plug is employed, it must be removed in order for the degraded tube to be available for repair or reconstruction.
Because the channel head is a relatively small area in which to work, it is required that whatever apparatus or device is used to remove the plugs from the plugged tubes in the tube bundle be capable of being operated in a relatively small space. In addition, because the channel head is radioactive, the device by necessity must be easy to manipulate and capable of removing plugs from plugged tubes in a relatively short period of time so as to prevent personnel from prolonged radiation exposure.
What is needed is a device or apparatus for removing plugs from plugged tubes within a tube bundle in a steam generator which is relatively small and easy to manipulate yet is capable of quickly removing plugs from plugged tubes, thereby reducing radiation exposure to maintenance personnel.